c# - 判断某月第一天、最后一天等的函数。

Question

我正在创建一个调度程序并且需要能够在 C# 中执行以下操作:

1. 找到 2012 年 6 月的第一个星期二
2. 找到 2008 年 3 月的最后一个星期五
3. 查找 2013 年 1 月的每个星期六
4. 找到 2009 年 7 月的第 3 个星期五
5. 查找 future 3 个月内的每个星期六
6. 查找 2018 年 3 月的每一天

返回的结果应该是 DateTime或 List<DateTime> .

Answer 1

以下是查找给定月份中第一个/最后一个指定星期几的方法:

public DateTime GetFirstDayOfWeekInMonth(int year, int month, DayOfWeek dayOfWeek)
{
    DateTime dt = new DateTime(year, month, 1);
    int first = (int)dt.DayOfWeek;
    int wanted = (int)dayOfWeek;
    if (wanted < first)
        wanted += 7;
    return dt.AddDays(wanted - first);
}

public DateTime GetLastDayOfWeekInMonth(int year, int month, DayOfWeek dayOfWeek)
{
    int daysInMonth = CultureInfo.CurrentCulture.Calendar.GetDaysInMonth(year, month);
    DateTime dt = new DateTime(year, month, daysInMonth);
    int last = (int)dt.DayOfWeek;
    int wanted = (int)dayOfWeek;
    if (wanted > last)
        last += 7;
    return dt.AddDays(wanted - last);
}

从中，您可以轻松找到其他问题的答案...只需添加 7 天即可找到您要查找的那一天的下一次出现

---

编辑:考虑更多，以扩展方法的形式使用它会非常方便，例如:

Console.WriteLine("Next monday : {0}", DateTime.Today.Next(DayOfWeek.Monday));
Console.WriteLine("Last saturday : {0}", DateTime.Today.Previous(DayOfWeek.Saturday));

这里是实现:

public static class DateExtensions
{
    public static DateTime Next(this DateTime from, DayOfWeek dayOfWeek)
    {
        int start = (int)from.DayOfWeek;
        int wanted = (int)dayOfWeek;
        if (wanted < start)
            wanted += 7;
        return from.AddDays(wanted - start);
    }

    public static DateTime Previous(this DateTime from, DayOfWeek dayOfWeek)
    {
        int end = (int)from.DayOfWeek;
        int wanted = (int)dayOfWeek;
        if (wanted > end)
            end += 7;
        return from.AddDays(wanted - end);
    }
}

它可能比我建议的第一种方法更灵活......有了它，你可以轻松地做这样的事情:

// Print all Sundays between 2009/01/01 and 2009/03/31
DateTime from = new DateTime(2009, 1, 1);
DateTime to = new DateTime(2009, 3, 31);
DateTime sunday = from.Next(DayOfWeek.Sunday);
while(sunday <= to)
{
   Console.WriteLine(sunday);
   sunday = sunday.AddDays(7);
}